1. Field of the Invention
The invention relates to a control apparatus and a method of controlling an internal combustion engine mounted on a vehicle.
2. Description of the Related Art
In an internal combustion engine mounted on a vehicle such as an automobile or the like, if the speed of the vehicle is equal to or below a predetermined speed to make a shift to idle operation in the process of stopping the vehicle from traveling, an idle speed control for adjusting the engine speed to a target engine speed during idle operation is performed. The target rotational speed used in this idle rotational speed control is variably set in accordance with the magnitude of a driving request for an auxiliary driven by the internal combustion engine. The target rotational speed is set higher as the driving request for the auxiliary increases. The target rotational speed is thus made variable in accordance with the magnitude of the driving torque for the auxiliary because as the driving torque for the auxiliary increases, the rotational resistance acting on the internal combustion engine during the driving of the auxiliary increases and the occurrence of a stall resulting from the increased rotational resistance during idle operation needs to be suppressed.
In the case where the driving request for the auxiliary is large when the speed of the vehicle becomes equal to or lower than the predetermined value to make a shift of engine operation to idle operation in the process of stopping the vehicle from traveling through the actuation of a brake, the target rotational speed for idle rotational speed control is set high, and the rotational speed of the engine during idle operation is also set high accordingly. In this case, since the rotational speed of the engine during idle operation is high as described above, the driving force applied to driving wheels of the vehicle is large. The rotational speed of the driving wheels is unlikely to be reduced even when a braking force is applied to the driving wheels by means of the brake. As a result, it takes some time to stop the vehicle from traveling.
To cope with this situation, it is conceivable to prevent a driving force from being applied to the driving wheels of the vehicle by holding a transmission in a neutral state as disclosed in Japanese Patent Application Publication No. 8-74992 (JP-A-8-74992) (paragraphs [0032] to [0034]) in the process of stopping the vehicle from traveling. In this case, even if the rotational speed of the engine is adjusted to the target rotational speed that has been set high when the speed of the vehicle becomes equal to or lower than the predetermined speed to make a shift of engine operation to idle operation, the driving force based on the rotation of the engine at that moment is not transmitted to the driving wheels. It is thus assumed that the vehicle can be swiftly stopped from traveling after the shift to idle operation. However, in the process of stopping the vehicle from traveling before the shift to idle operation, the transmission is held in the neutral state to shut off a transmission path of the driving force between the driving wheels and the internal combustion engine. As a result, the rotational resistance of the internal combustion engine does not serve as a braking force for the driving wheels. It is therefore demanded to ensure the braking force for swiftly stopping the vehicle from traveling by using the brake alone. However, such a braking force cannot always be ensured by using the brake alone. It takes some time to stop the vehicle from traveling when the braking force cannot be ensured with ease.
Further, instead of holding the transmission in the neutral state in the process of stopping the vehicle from traveling, it is also conceivable to reduce the driving request for the auxiliary to reduce the target rotational speed for idle rotational speed control upon a shift of engine operation to idle operation, reduce the rotational speed of the engine during idle operation on the basis of the reduced target rotational speed, and hence make the driving force applied to the driving wheels small on the basis of the rotation of the engine. In this case, in the process of stopping the vehicle from traveling before the shift to idle operation, the rotational resistance of the internal combustion engine serves as a braking force for the driving wheels. Thus, the braking force for swiftly stopping the vehicle from traveling is applied to the driving wheels through the rotational resistance of the internal combustion engine and the brake. Further, after the shift of engine operation to idle operation in the process of stopping the vehicle from traveling, the drive request value for the auxiliary is reduced to reduce the target rotational speed used for idle rotational speed control. The driving force applied to the driving wheels can thereby be made small. Owing to the foregoing procedure, the vehicle is swiftly stopped from traveling.
As described above, when a shift of engine operation to idle operation is made in the process of stopping the vehicle from traveling, the drive request value for the auxiliary is reduced to thereby reduce the target rotational speed for idle rotational speed control. Thus, the vehicle is swiftly stopped from traveling.
In normal idle rotational speed control, however, a predetermined delay time is set between a time point corresponding to reduction of the drive request value for the auxiliary as described above and a time point corresponding to actual reduction of the target rotational speed. Accordingly, when the drive request value for the auxiliary is reduced, the target rotational speed is not reduced until the lapse of the predetermined delay time from a time point corresponding to the start of reduction of the drive request value for the auxiliary. The target rotational speed is reduced after the lapse of the delay time.
This delay time is set because of the following reason. That is, when the drive request value for the auxiliary is reduced, the rotational speed of the engine during idle operation may be reduced with the drive rate of the auxiliary not having been reduced completely, unless the target rotational speed is restrained from being reduced in response to the reduction in the drive request value for a time needed (equivalent to the delay time) to ensure a reduction in the drive rate of the auxiliary resulting from the reduction in the drive request value. If the rotational speed of the engine during idle operation is reduced with the drive rate of the auxiliary not having been reduced completely, reduction of the rotational speed of the engine occurs with high rotational resistance for driving the auxiliary, which acts on the internal combustion engine. At this moment, there is a disturbance acting on the driving wheels in such a direction as to stop rotation thereof. For example, an external force (a frictional force or the like) from a road surface side is applied to the driving wheels reversely to the rotational direction thereof. In that case, the internal combustion engine may undergo a stall as a result of a further reduction in the rotational speed of the engine caused by the disturbance. The occurrence of a stall of the internal combustion engine in a situation as described above is suppressed by setting the delay time.
However, when the predetermined delay time is set between the time point corresponding to reduction of the drive request value for the auxiliary and the time point corresponding to reduction of the target rotational speed for idle rotational speed control in response thereto upon a shift of engine operation to idle operation in the process of stopping the vehicle from traveling, the time needed to stop the vehicle from traveling is prolonged by the delay time. As a result, it is difficult to swiftly stop the vehicle from traveling. Further, it is also conceivable to shorten the delay time giving higher priority to the swift stoppage of the traveling of the vehicle than to the suppression of the occurrence of a stall of the internal combustion engine. In this case, however, the possibility of the internal combustion engine undergoing a stall inevitably becomes high.